The Skeletal Biology Program has focused on biochemically characterizing osteogenic cells, including bone marrow stromal cells (BMSCs), a heterogeneous population of clonogenic cells that give rise to osteoblasts/chondrocytes, adipocytes and hematopoiesis supportive stroma. Studies were designed to test the hypothesis that different members of the stromal cell population represent pluripotential cells, while others are more committed to particular phenotypes. This hypothesis was tested by determining the ability of individual clones of human BMSCs to form a bone/bone marrow organ by using a newly developed in vivo transplantation system. It was found that 54% of the clones studied supported bone formation, but only half of these clones supported bone formation and hematopoiesis. These results prove that some, but not all, of the stromal cell population maintain their ability to form bone, hematopoiesis supportive stroma and associated adipocytes. The differences between non-bone forming clones, clones that form bone and those that form bone and support hematopoiesis were further investigated by examining their expression of receptor tyrosine kinases (RTKs), a family of receptors that has been implicated in the proliferation and differentiation of many connective tissue cells. PDGF-receptor beta, EGF receptor, FGF receptor-1 and Axl were identified in the multi-colony derived population of BMSCs. Clonal populations of BMSCs were found to express varying levels of these four RTKs. Although not predictive a particular clone's ability to support bone formation in an in vivo transplantation assay, it was found that there is relatively high levels of PDGF-R (beta) in bone-forming clones, and relatively high levels of EGF-R in non-bone forming clones, and that the rate of proliferation of bone-forming clones is positively correlated with the amount of bone formed in vivo. Members of the program also collaborated with other Branch members and outside investigators in the establishment of immortalized BMSCs from a patient null for the expression of the estrogen receptor (alpha), the identification of SHOX/PHOG, a transcription factor implicated in skeletal growth in BMSCs, and characterization of human cementoblasts in an in vivo transplantation system.